Well I can't comment on your import issues, but the Screensaver is not broken in 10.7 on my machine.
As for reverting:
Unless you have an iPhoto Library that you've been updating then you've got a lot of work ahead of you
iPhoto has no knowledge of, and knows nothing of how the Aperture Library works. It cannot read the Aperture library.
1. Export all your Masters
2. Export all your Versions
You can write the metadata to the files on export - check out the Export presets. Note that you cannot export Faces. There is no convention for sharing Faces. The best you can do is use Faces as a basis for keywording.
Import them into iPhoto. Note that there is no way to associate the masters with the versions in iPhoto, so you'll have apparent duplicates.
When attempting to import raw from the Canon 7D it reports that some pictures are an supported format and asked where (outside Aperture) is like to put them. It will now only import jpg or movie files. I've previously imported 10s of thousands of raw (CR2) files. There are other references on the web to raw import being broken, so I'm not the only one seeing the problem.
Interesting, screensaver broke for me the moment I updated to Lion and hasn't worked since.
I configure the screensaver (as it was pre Lion) to use either an album or a smart album - the screensaver config shows the list and hierarchy of albums, but having chosen one, on either test or when the screensaver kicks in I just get an error message about there being no pictures.
Is your use of screensaver the same?
As it happens mine is pointed at a Project rather than an Album, but I just tested with an Album and it worked ok.
Try with a project.
If you're using the Mosaic option you can get messgae "Looking for Pictures" as it build caches - the Saver does arrive, and you see less and less of thatr message as time goes by.
Hedgert, do you have access to Aperture version 3.1.3 through Time Machine or any other backup device? If so, why not try downgrading from version 3.2 rather than moving to iPhoto? Lots of discussion in this forum about version 3.2 crashing when loading RAW files plus some information about how to go back to version 3.1.3 and restore your Aperture library. I, myself, was a victim of the upgrade and have since followed the steps provided by others and successfully restored everything. I did have a problem with the vault - housed in a partition of on an external HD - which was damaged beyond repair by Disk Utility; I solved that problem by erasing the partition dedicated to the vault and replacing all of the photos, albums etc. Now everything is working well. BYW, I am still on OS 10.6.8 - and will likely stay there with version 3.1.3 awhile until Apple finds a solution to the crashing problem.
An update to Aperture 3.2.1 has now been released and this has fixed the Canon 7D raw issue that I mentioned above - so obviously other users faced the same issue I found and Apple have acted.
The bug with screen saver continues despite this update - which is disappointing - right from the first release of Lion.